A Primer on Water Quality: Pollutant Pathways

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 Background information
Contaminants such as sediment, nutrients (nitrogen, phosphorus), herbicides, insecticides, salts and animal wastes can reach surface water and ground water in runoff or by leaching. Runoff refers to water flowing over the ground surface including rainwater, snowmelt or irrigation water that does not infiltrate the soil. Leaching is the process whereby soluble materials are carried downward through the soil profile by snowmelt, rain or irrigation water. Pollutant transport is influenced by the chemical and physical properties of the pollutant.

Contaminant transport is influenced by the hydrologic cycle (Figure 1) (The hydrologic (water) cycle. (A primer on water questions and answers. Environment Canada, Conservation and Protection 67 p.) Water is supplied from rainfall, irrigation or snowmelt. The rate of supply relative to the rate of infiltration into the soil will determine how much surface runoff, if any, occurs. Storm intensity affects the rate of water supply. Furthermore, the capacity of the soil to store water determines how much water moves through the soil profile as subsurface drainage (subsurface flow and leaching). Factors that influence infiltration also influences the relative amounts of surface runoff and subsurface drainage. For example, fine-textured soils with naturally lower permeabilities (i.e. clay) generally generate more surface runoff from a given storm event than a coarse textured soil (i.e. sand). Soil moisture content prior to a storm event will also affect the infiltration rate of the water.

Sediment is usually transported in surface runoff, whereas chemical pollutants are transported attached to sediment or dissolved in surface runoff and subsurface drainage (leaching). The ability of the pollutant to adsorb onto sediment will influence the way it is transported. Soluble chemicals (chemicals that do not adsorb onto soil particles (e.g. nitrate, atrazine)), tend to leach through the soil profile. However, low infiltration rates can enhance surface runoff of soluble pollutants. In contrast, the adsorption of the chemical onto soil particles can prevent the chemical from leaching into the soil profile. Microorganisms and organic matter are also carried in surface runoff.

The presence of macropores, such as earthworm or other animal burrows, plant root channels, cracks and fissures, influences the transport of pollutants. A system of continuous, often surface-connected soil macropores allows incoming water and dissolved chemicals to infiltrate into the soil profile in a process called preferential flow. Preferential flow results in chemicals leaching deeper and faster to aquifers.

Sediment is transported by water or wind through erosion and sedimentation processes. The detachment of soil particles by the impact of raindrops and water movement over the soil surface results in erosion. Sediment is carried by runoff and transported varying distances downstream depending on particle size. Coarser sediments deposit first, while finer sediments move further. Deposition or sedimentation of sediment occurs at many sites including the bottom of a hill slope; the edge of a field or in a windbreak, lake or reservoir; and on a floodplain. Erosion and sedimentation processes can transport sediment in one large event or a succession of smaller events from the land to surface water.

Raindrops cause soil detachment by breaking down soil aggregates into small particles that are easily transported in shallow overland flow to rills. Sheet erosion occurs rather uniformly over the slope whereas rill erosion occurs when surface runoff concentrates in small, well-defined channels. Gully erosion forms gullies, large channels that cannot be smoothed by regular tillage practices.

The rate and magnitude of water erosion are influenced by rainfall intensity, water runoff, the susceptibility of soils to erosion (erodibility), slope gradient and length, and vegetation cover. Erosion due to rainfall is usually greatest during short-duration, high-intensity thunderstorms; during snowmelt; when soils have high moisture content; and when vegetative cover is minimal.

Soil erodibility depends on soil properties that include soil structure and texture, infiltration rate and organic matter content. Silt and clay textured soils are highly erodible because of reduced infiltration due to poor soil structure and low organic matter content. Conversely, sand and loam textured soils are less erodible because of improved soil structure from higher organic matter content which facilitates infiltration.

Erosion can also be affected by watershed slope and vegetation cover. Steep surfaces are more likely to erode than gently sloping terrain. Vegetative cover protects the soil from erosion and slows down the flow of surface runoff.

Organic matter
All substances of animal or plant origin contain carbon compounds and are, therefore, organic (USDA 1992). Animal wastes and dying vegetation produce organic material which is generally degraded or decayed by microorganisms (e.g. bacteria) to simpler compounds, either other forms of organic matter or nonorganic compounds such as nitrogen or phosphorus. These compounds can then be incorporated into the soil. Problems occur, however, if high concentrations of organic material on the soil surface are carried by runoff or deposited directly into receiving streams before the biodegradation process can take place. This can occur when heavy rains wash away fresh organic wastes, or during spring runoff when over-winter accumulations of organic material are flushed into receiving streams in runoff. Wastes accumulated over the wintering season can not biodegrade because of freezing temperatures. Further, wastes can be directly deposited into lakes and rivers if livestock have direct access to these water sources. Generally, water quality problems from organic material result from the decomposition process.

Since bacteria require oxygen to decompose organic material, large quantities of dissolved oxygen can be consumed when organic material is added to streams. A rapid increase in bacterial populations results in a drastic reduction in dissolved oxygen in a stream. The point in a stream where the maximum oxygen depletion occurs can be considerable distance downstream from the point where pollutants enter the stream. The level of oxygen depletion depends primarily on: the amount of waste added; the size, velocity, and turbulence of the stream; the initial dissolved oxygen levels in the waste and in the stream; and the temperature of the water (USDA 1992). Dissolved oxygen is essential for the survival of aquatic organisms. Adding organic waste to a stream can lower oxygen levels so that fish and other aquatic life are forced to migrate from the polluted areas or die from a lack of oxygen. The decomposition of organic material can also create undesirable colour, taste and odour problems in lakes and rivers used for public water supplies (USDA 1992).

Inorganic nitrogen in the form of nitrate (NO3-) and ammonia, present as the ammonium ion (NH4+) is an important plant nutrient in agricultural and biological ecosystems. Nitrate and ammonia nitrogen are transported to surface and ground waters by leaching and runoff. Many studies show that residual nitrogen in cropland soils is closely related to the amount of fertilizer, manure or legume nitrogen applied to the soils. Excessive nitrogen application leaves a pool of residual nitrogen in the soil at the end of each growing season. This residual nitrogen may pollute ground or surface water. Most of the residual nitrogen is nitrate as soil microbes can convert more stable forms of nitrogen (i.e. ammonium) to nitrate. Nitrates and ammonium salts (found in some commercial fertilizers) are highly soluble in water and can move easily through the soil profile whereas ammonia nitrogen is generally transported to surface waters attached to eroded soil particles and organic material.

Nitrogen is an important plant nutrient and is one of the most mobile compounds in the soil-crop system. Nitrogen is continually cycled among plants, soil organisms, soil organic matter, water and the atmosphere (Figure 2) (The nitrogen cycle. (adapted from National Research Council 1993)). The flux of nitrogen into, out of, and within the soil takes place through complex biochemical and microbial transformations. The balance between inputs and outputs and the various transformations in the nitrogen cycle determine how much nitrogen is available for plant growth and how much may be lost to the atmosphere, surface or ground water. The main processes that affect nitrogen transport include: mineralization, nitrification, denitrification, immobilization, and volatilization.

Mineralization transforms nitrogen in soil organic matter to ammonium ions (NH4+), releasing them into the soil. Ammonium is relatively immobile in the soil, being strongly adsorbed to clay minerals and organic matter. Ammonium is readily converted by soil microbes and bacteria, first to nitrite (NO2-) and then to nitrate through nitrification. Nitrate is soluble and can leach below the rootzone and into ground water if it is not assimilated by growing plants, or can be lost to the atmosphere through volatilization. Nitrate in ground water can move through springs, seeps or shallow flow systems to recharge surface water or they can leach into deeper aquifers. Denitrification, however, converts nitrate to nitrogen gas (N2) which is released into the atmosphere or nitrous oxide (N2O) which is quite soluble in water and may be washed out of the soil in leachate. Immobilization by microorganisms converts ammonium and nitrate to organic nitrogen in the microorganisms, which is temporarily not available for plant uptake. Immobilized nitrogen can subsequently be re-mineralized back to ammonium. Nitrogen can also volatilize directly from field-applied fertilizers and manure. Direct volatilization losses can be quite large, especially from surface applications of manure, contributing nitrous oxides, ammonia and methane to the atmosphere.

Nitrogen transformation processes are affected by the availability of oxygen and organic carbon plus the presence of microorganisms such as Nitrosomonas and Nitrobacter in the soil. These processes can go on simultaneously, coexisting in close proximity, and varying temporally in the same setting. It is the balance between these processes and their seasonal timing that determines how much nitrogen is available for crops and how much nitrogen may be lost from the soil to the atmosphere or surface and ground water.

Most phosphorus is lost from agricultural lands through surface runoff. Phosphorus can be in solution (soluble phosphorus) or bound to eroded sediment particles or organic material (particulate phosphorus). Separation between soluble and particulate phosphorus is usually determined through filtration with soluble phosphorus passing through a 0.45um filter. Soluble phosphorus is more readily available to crops, algae and aquatic plants; however, particulate phosphorus can be a long-term source of phosphorus in the aquatic ecosystem. Phosphorus is strongly adsorbed onto soil particles and therefore, phosphorus contamination of ground water is generally not a problem. Most of the phosphorus transported to surface waters is attached to eroded soil particles, particularly from fine-textured soils (e.g. clay and silt) near watercourses.

Phosphorus is an essential plant nutrient. Like nitrogen, phosphorus added to the soil in chemical fertilizers or animal wastes, goes through a series of transformations as it cycles through plants, animals, microbes, soil organic matter and the soil mineral fraction (Figure 3) (The phosphorus cycle (source Marston 1989)).

Unlike nitrogen, particulate phosphorus is tightly bound to soil particles. In acidic mineral soils (pH < 6.5), phosphorus is bound to iron and aluminum oxides which retain phosphorus near the surface of the soil. In more neutral or alkaline soils (pH > 6.5), phosphorus is bound to calcium forming apatite. In soils that lack iron and aluminum oxides (i.e., organic or sandy soil), phosphorus can leach more readily into the soil profile.

Most soil phosphorus is found as a complex mixture of inorganic and organic materials. Organic material such as plant or crop residues and animal manure are broken down by soil microorganisms through mineralization. Following decomposition, some organic phosphorus is released as phosphate ions that are immediately available for plant uptake. If the phosphate ions are not assimilated by growing plants, they can adsorb onto soil particles or be immobilized thus becoming less available for plant uptake. Most phosphorus lost from agricultural land is adsorbed onto eroded soil particles thus the transport processes are similar to the those described for sediment. However, soluble phosphorus losses from agricultural land can be substantial.

Pesticides: herbicides and insecticides
Pesticides (herbicides, insecticides and fungicides) are synthetic organic compounds. They are transported to water bodies through direct application, runoff, leaching, aerial drift and atmospheric deposition. Pesticide transport depends on the following: the physical/chemical properties of the pesticide; the method and timing of its application; weather and climatic conditions; and land characteristics. Adsorption onto soil particles and rate of degradation have a strong influence on pesticide transport.

Pesticides are applied as liquids or solids through a variety of methods such as aerial or canopy spraying, incorporation or injection into the soil, and mixed with water and sprayed on the soil or plants. Unlike nitrogen or phosphorus, there is no inherent pesticide cycle comparable to the natural nutrient cycles. Pesticides can be taken up by plants, adsorbed onto plant surfaces, broken down by sunlight (photodegradation) or ingested by animals, insects, worms or microorganisms in the soil. Pesticides may vaporize and enter the atmosphere (volatilization) or break down via microbial and chemical pathways into byproducts (biological degradation). These pesticide transformation byproducts can be less, more or similar in toxicity when compared to the parent pesticide (Day 1990). Some pesticides are also removed when the crop is harvested. Pesticides may leach through the soil profile and be either immobilized by adsorbing onto organic matter and clay minerals or dissolve in soil water. Pesticides can leach out of the root zone with rain or irrigation water or be transported with surface runoff (Figure 4) (Environmental fate of pesticides after application (adapted from Jacobson and Johnson.1993)) .

The chemical properties which influence the transport of pesticides are persistence and adsorption. The ability of a chemical pesticide to remain active in the environment is termed persistence and is measured according to its half-life, the time it takes for 50% of the chemical to be degraded or transformed. Most pesticides degrade or transform over time as a result of several chemical and microbiological reactions. Chemical degradation includes such reactions as photolysis (photochemical degradation) and hydrolysis (reaction with water). Biological degradation is the break down of a chemical by soil microbes. Generally, chemical reactions result in only partial transformation of pesticides whereas soil microorganisms can completely break down pesticides to carbon dioxide, water and other inorganic constituents. Most microbes occupy the root zone therefore; pesticides that have leached below this depth are less likely to be microbially degraded and are more likely to persist.

Adsorption is the process that binds pesticides to soil particles. Pesticides that dissolve readily in water are considered to be highly soluble (e.g. 2,4-D, dicamba, dinoseb, MCPA and molinate). Soluble chemicals can leach through the soil profile or be transported in runoff. Pesticides are likely to contaminate ground water if: they have a low adsorption capacity; their persistence is long; and their water solubility is high (e.g. alachlor, aldicarb, atrazine, bromacil, carbofuran, cyanazine, metolachlor and simazine). Conversely, pesticides that are strongly adsorbed to clay and organic matter are transported by erosion and overland runoff (e.g. DDT, dieldrin, endosulfan, toxaphene, lindane, heptachlor, chlordane and difocol) and therefore can affect surface water supplies.

Pesticides with high vapour pressures are easily volatilized to the atmosphere. In addition, pesticides are lost through gaseous diffusion from the soil surface after application. However, some highly volatile pesticides have been shown to move down the soil profile into groundwater aquifers.

Soil permeability, as defined by soil texture, structure and organic matter content also influences the transport of pesticides. Soil permeability is a measure of how fast water can move through the soil profile. In general, the infiltration rate and permeability of coarser-textured soils are greater than those of finer-textured soils. A chemical that can infiltrate into the soil is less likely to be lost in surface runoff but is more likely to leach into ground water. Soil structure reflects how soil particles are aggregated and cemented together. Poorly structured soils (e.g. clayey) are easily eroded and have lower infiltration rates; therefore, pesticides that are adsorbed onto these soils are transported in surface runoff. Macropores and cracks in the soil profile allow pesticides to move downward through the soil profile more quickly. Pesticides adsorb onto organic matter; therefore, pesticide mobility and contamination of ground water are greater in soils with low organic matter content.

Salts and trace elements
Naturally occurring salts and trace elements in the soil can be transported to surface or ground waters with rain or irrigation water percolating through the soil profile. Salts and trace elements can become concentrated on the soil surface when irrigation water contains high levels of salts. Therefore, surface runoff can also transport soluble salts and trace elements to surface waters when draining saline soils. The main sources of salts and trace elements include irrigation water from return flows, soil and mineral weathering, fertilizers, crop residues and animal manure. High levels of salts and trace elements decrease crop production and can be detrimental to animals consuming these plants (CAST 1992).

Other Documents in the Series

  A Primer on Water Quality
A Primer on Water Quality: Agricultural Impacts on Water Quality
A Primer on Water Quality: Agricultural Contaminants - Background Information
A Primer on Water Quality: Impact of Crop Production Practices on Water Quality
A Primer on Water Quality: Impact of Livestock Production Practices on Water Quality
A Primer on Water Quality: Pollutant Pathways - Current Document
A Primer on Water Quality: Pollutant Processes in Rivers and Lakes
A Primer on Water Quality: References
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This document is maintained by Laura Thygesen.
This information published to the web on March 4, 2002.
Last Reviewed/Revised on August 22, 2017.